Question

The increasing order with respect to dipole moment of the following molecules is: align* BF_3, H_2O, NF_3, NH_3 align*

• A. \(NF_3 \lt BF_3 \lt NH_3 \lt H_2O\)
• B. \(NF_3 \lt BF_3 \lt H_2O \lt NH_3\)
• C. \(BF_3 \lt NH_3 \lt H_2O \lt NF_3\)
• D. \(BF_3 \lt NF_3 \lt NH_3 \lt H_2O\)

Hint:

Remember these standard dipole moment trends: \[ \begin{aligned} BF_3 &: \text{Symmetrical, } \mu=0 \\ NF_3 &: \text{Bond moments oppose lone-pair moment} \\ NH_3 &: \text{Bond moments reinforce lone-pair moment} \\ H_2O &: \text{Bent structure with high resultant dipole moment} \end{aligned} \]

Answer 1

The Correct Option is D

Concept: Dipole moment depends on both bond polarity and molecular geometry. It is the vector sum of all bond moments present in a molecule.

• Symmetrical molecules may have zero dipole moment due to cancellation of bond moments.

• Lone pairs influence molecular geometry and hence the resultant dipole moment.

• In \(NF_3\), the bond moments oppose the lone-pair moment, reducing the net dipole moment.

• In \(NH_3\), the bond moments and lone-pair moment act in the same direction, increasing the dipole moment.

Step 1: Determine the dipole moment of \(BF_3\). \(BF_3\) has a trigonal planar geometry and is perfectly symmetrical. \[\begin{aligned} \mu(BF_3)=0 \end{aligned}\] Thus, \(BF_3\) has the lowest dipole moment.

Step 2: Compare \(NF_3\) and \(NH_3\). \[ \begin{aligned} NF_3 &: \text{Lone pair opposes bond moments} \Rightarrow \text{Small dipole moment} \\ NH_3 &: \text{Lone pair adds to bond moments} \Rightarrow \text{Larger dipole moment} \end{aligned} \] Hence, \[\begin{aligned} \mu(NF_3) \lt \mu(NH_3) \end{aligned}\]

Step 3: Compare \(NH_3\) and \(H_2O\). Water possesses two lone pairs and a bent geometry, leading to a greater resultant dipole moment. Typical values are: \[ \begin{aligned} BF_3 &:\quad 0 \ \text{D} \\ NF_3 &:\quad 0.24 \ \text{D} \\ NH_3 &:\quad 1.47 \ \text{D} \\ H_2O &:\quad 1.85 \ \text{D} \end{aligned} \] Therefore, \[\begin{aligned} BF_3 \lt NF_3 \lt NH_3 \lt H_2O \end{aligned}\]

Step 4: Identify the correct increasing order. \[\begin{aligned} \boxed{BF_3 \lt NF_3 \lt NH_3 \lt H_2O} \end{aligned}\] Hence, option \(\mathbf{(D)}\) is correct.